The present invention relates to a connecting structure for a refractory metal member difficult to connect by, e.g., welding, a metal member connecting method, a ceramic heater, and a ceramic heater manufacturing method.
A known example of a glow plug and the like for ignition of various heaters and combustors and auxiliary starting of a diesel engine uses a ceramic heater made from a high-temperature heating ceramic sintered body.
For example, Japanese Patent Laid-Open No. 55-126989 discloses a ceramic heater of this type in which a refractory metal such as tungsten (W) or a tungsten rhenium alloy (W--Re) is used as a heating element and buried in a silicon nitride insulating ceramic that is excellent in oxidation resistance and thermal shock resistance.
In this conventional ceramic heater, its distal end portion must function as a high-temperature heating portion. For this purpose, a metal member serving as a heating element is arranged at the distal end portion of the heater. One end of the metal member is wound around one end of a lead, or a straight portion of one end of the lead is inserted in a coiled portion of the metal member. The metal member and the lead are put in a ceramic powder and sintered to bury them in the ceramic sintered body.
In the ceramic heater manufactured by the above method, the electrical connection becomes unstable at the connecting portion between the metal member and the lead when the metal member serving as a heating element is wound around the straight portion of the lead or the straight portion of the lead is inserted in the coiled portion of the metal member in connecting, to the lead, the metal member serving as a heating element to be buried in the ceramic sintered body. The above winding operation makes automatic assembly of the metal member and the lead difficult. Accordingly, they are manually assembled, resulting in high cost.
In the use of a refractory metal, a connecting method using another member such as brazing cannot be employed. The refractory metal cannot be connected by welding of melting only a base material and joining the refractory metal. There is no joining means other than the above one, so any measure must be taken.
For example, Japanese Patent Laid-Open No. 58-110919 discloses a ceramic heater in a diesel engine glow plug in which a metal member serving as a control element for controlling a supply current to the heating element is series-connected to a metal member functioning as a heating element at the distal end portion of the heater, and buried in a ceramic sintered body. Further, another ceramic heater in which two types of metal members serving as control elements like the one described above are series-connected to more effectively control the supply current to the heating element is also known.
In the ceramic heater having such a structure, however, the above problem about joining of the metal member becomes more serious because a plurality of types of metal members serving as heating and control elements must be series-connected to each other and connected to leads. Particularly, since the ceramic heater uses a refractory metal member, a stable connecting state of the refractory metal by welding is difficult to obtain.
The above-described connection of metal members is required in not only the ceramic heater but also various electrical devices, and the above problems must be considered. For example, in connecting, to a lead, a refractory metal wire such as the filament of a light bulb or electron microscope, the same problem as in the ceramic heater also arises, and any measure must be taken.